1. Field of the Invention
The invention relates to a transparent heat protection element having at least one support element and at least one protective layer comprising a reaction product containing aqueous alkali metal silicate solution and aluminate- or borate-modified silicon dioxide and also a process for producing it.
2. Description of Related Art
DE-A 2 414 575 describes fire-shielding glass panes containing a fire-resistant layer comprising a polymer membrane which on at least one side has a layer containing a barrier-forming material. This material may be an aluminate or an alkali metal silicate. The purpose of this material is to make the layer(s) more opaque to infrared radiation on heating, since this material forms thermally insulating barriers.
U.S. Pat. No. 4,190,698 describes transparent fire protection panes containing at least one dried layer comprising hydrated alkali metal silicates and one or more auxiliaries such as urea, polyhydric alcohols, colloidal silica or sodium aluminate. When colloidal silica and sodium aluminate are used, a modulus of less than 4 is achieved. The purpose of the addition of the auxiliaries is to improve the resistance of the layer and thus the fire protection pane in the case of fire. A disadvantage in the production of these fire protection panes is that evaporation of water from the original formulation is necessary, which is complicated from a production engineering point of view. The dried fire protection layer is formed by casting onto an open surface and subsequent drying and cannot be formed in closed hollow spaces. It is also known that the addition of sodium aluminate to the protective layer leads to precipitates and thus, as a further disadvantage, to the protective layer rapidly becoming cloudy.
WO-A 2004/014813 describes the production of heat protection elements based on a solution comprising water glass and an aluminate, with the aluminate having to be partially neutralized using organic acids such as citric acid before addition to the silicate solution. If partial neutralization of the aluminate is not carried out, a stable solution of water glass and aluminate is not obtained. A disadvantage in the production of these heat protection elements is that evaporation of water from the original formulation for the heat protection elements is necessary, which is complicated in terms of production engineering.
WO-A 94/04355 describes the production of a transparent heat protection element having at least one support element and a protective layer composed of water-containing alkali metal silicate. Such transparent heat protection elements are employed, for example, for the production of fire protection glasses. A significant constituent of the heat protection elements is the protective layer of water-containing alkali metal silicate which is produced by reaction of an alkali metal silicate and a silica aqua sol in such a mixing ratio that the molar ratio of silicon dioxide to the total amount of alkali metal oxide, known as the modulus, is greater than 4:1. The silica sol component is in this case used as hardener for the alkali metal silicate. Curing by means of silica sols enables the evaporation of water from the original formulation of the layer, which is necessary in other known protective layers and is complicated in terms of production engineering, to be dispensed with.
However, a disadvantage of protective layers composed of water-containing alkali metal silicate and silica sol as hardener, as described in WO-A 94/04355, is their tendency to become cloudy during use. This applies particularly in the case of prolonged exposure to temperatures above 20° C., which can be prolonged during the summer months and can accelerate the clouding process.
Electron micrographs confirm that the clouding of the aqueous alkali metal silicate protective layer is caused by a crystallization process. The additions of oxides of the elements of main group III of the Periodic Table, in particular boron oxide or aluminum oxide, are known to promote crystallization processes in glasses and should accordingly be expected to accelerate the clouding of glasses.
There is therefore a need for transparent heat protection elements and also materials for use as protective layers in such heat protection elements, which have a high transparency and aging resistance. In addition, the starting composition for the protective layer should be flowable and be suitable for casting into hollow spaces and subsequently harden over an appropriate period of time to form the protective layer.